1. Field of the Invention
This invention relates to high speed semiconductor devices and more particularly to a heterojunction transistor with an ultra-thin base region which can be heavily doped to reduce the base resistance.
2. Description of the Prior Art
In U.S. Pat. No. 4,173,763, entitled "Heterojunction Tunneling Base Transistor" issued to Leroy L. Chang, et al. on Nov. 6, 1979, there is disclosed a heterojunction semiconductor device having emitter, base and collector regions separated by heterojunctions with the base region having a thickness in the order of 100 A so that charge carriers can tunnel therethrough. One of the characteristics of this device is that the energy gaps of the emitter and collector regions are shifted in the same direction with respect to the energy gap of the base region and overlap with the energy gap of the base region, i.e. both the valence band and the conduction band in the emitter and collector regions are shifted in the same direction with respect to the valence band and conduction band of the base region. This provides a conduction band edge discontinuity which establishes a potential barrier against charge transfer from the emitter to the base region. Additionally, the band gap structure as described provides a valence band discontinuity which isolates the base region from both the emitter region and the collector region, meaning that majority carriers in the base region are trapped in a potential well so that they cannot easily move to either the emitter or collector for recombination with opposite type charge carriers. Moreover, the base region is heavily doped which means that its base resistance is low.
In the present invention, a heterojunction transistor is described which does not use the tunneling mechanism mentioned above. A special treatment of the junction interface is involved which, combined with the use of a wide-gap emitter and heavily doped, ultra-thin base layer, results in ultra-high speed operation.
Accordingly, it is an object of the present invention to provide an improved high speed semiconductor device of the heterojunction type.